Angels are genderless and ageless, humans are men and women, fallible and mortal. The subordination of these two species is timeless, indisputable and unbreakable.
But when an angel takes a human vessel, it becomes vulnerable. It knows humanity, but never experienced its needs and weaknesses.
When Castiel took Jimmy Novak's vessel, his movements were gawky and imbalanced, but soon he perceived how to use the mortal's knowledge. The body was strange and quaint, the clothes were tight and uncomfortable. Then he slowly get used to the involuntary movements, the feelings of temperature on his skin, and the touch of cold water.
Long months passed, and Castiel thought he knows everything about the human body, it's functions and utilities. But when his skin first touched skin, he suddenly got short of breath, he started shaking, because the feeling was so new. Rough fingers touched his hands, and vibrating heat ran up his arm from his wrist, it reached his spine, and the body's pulsating engine became hostility loud and harsh. The experience was scary and annoyingly beautiful; he thought so far that the skin is to protect the inside from the effects of the outside, but yet this subtle touch intruded his defense system. He was disturbed, and that night he returned to the musty motel room to look at the hand that tossed him into vagueness.
Dean Winchester was lying in wakeful dream, a firearm under his right hand, polished. Castiel stood beside the bed, he felt the flowing air, he was staring at the palm cracky from fight and full of scars. It didn's seem different from other hands, it didn't show it's narcotic power. Castiel thought that he has to touch it, get to know it. His sudden decision made him move, he reached his right hand to the sleeping man's, but didn't touch it yet. He was so close, felt the electricity, he was shivering, and the unknown feeling was about to born again. He forgot to breath, and when his lungs plaintively let in the sudden amount of air, his eyes gloomed. It was just a tiny movement.
A fragment of moment later he felt his wrist pinched to the bed, and the cold iron of a hard barrel pressed to his chest. He looked down, and the burning green eyes of Dean Winchester were scanning him discomposedly. Castiel wanted to say something, but somehow he couldn't. He just stared into the eyes that promised absolution and apostasy at the same time, staring at him wiht a questioning look.
The silence was trembling between them, the moments went by, the centimeters was shortening, and when Castiel's lips first touched lips, the madness tearing apart his insides was unbearable. It was catharsis, maybe because of the gun still pointing at his heart, or not, but he suddenly realized that there's no skin that can shut this out.
The gun quietly touched the ground.
When wet lips touched his cold skin, hard, cracked fingers grasped his hair and white teeth bit his tensing abdominal muscles, his numb mind wanted to tear his skin, this awful prison that did not let him to unfold his dark wings. He lost his ethereal angelic being, he understood the fall, finally understood humanity's all dirt and miracles. He fell into heated collapse after the excruciating pleasure, and he knew that he likes being human, he wants to be human. To sin is good and to sin is ought, and the sin is sweet and releasing.